I think that they are making the same mistake they made with the Shuttle: too much payload. The Shuttle's original payload requirements were driven by military payloads that eventually didn't end up flying on the Shuttle anyway.

As far as I can see, SLI is also working around Shuttle-sized payloads. Bad idea. They need to whip up something that can easily and cheaply take about 10,000 lbs. max into orbit. Larger payloads can be launched by either the old Shuttle, or expendables. I don't have a specific design in mind, although a winged booster using conventional jet engines and an orbiter on top would work quite well, and be a lot faster to develop than an SSTO.

I predict the SLI thing will end up pretty much the same way as the X-33, but I hope I'm wrong.

Long before they worked up SLI, there were some things they could have done for the Shuttle first. Top priority should have been replacement of the solids with throttleable liquid boosters. A cargo version, i.e. an expendable cargo bay mounted in the orbiter's place with a recoverable engine pod at the back, would have been a really good idea too.